Feed-back circuit



May l'2, 1942.

C. S. ROOT FEED-BACK CIRCUIT Filed July 26, 1938 AUDIO AUDIO E AMPLIFIER Inv n'bor Charles S.Rooc, b

His Attorney- Patented May 12, 1942 FEED-BACK omcmr Charles S. Root, Stratiord, Com, assignor to General Electric Company, a corporation of New York Application July 26, 1938, Serial No. 221,319

10 Claims.

My invention relates to feed-back circuits for electron discharge devices, and more particularly ent application is assigned, is disclosed an audio amplifier employing a degenerative feed-back system in which the magnitude of the feed-back voltage is automatically reduced upon movement of the manual volume control to increase the ap plication of signal electromotive force to the amplifier. In this way, when the manual volume control is in a position for maximum sensitivity to voltages to be amplified, as when used in a radio receiver, the degeneration is automatically reduced.

One of the objects of my invention is to provide certain improvements upon systems of the type described in said Bachman application and more particularly to provide means whereby the feed-back voltage may be reduced to zero upon a certain adjustment of the volume contrql for high sensitivity. A further object of the invention is to provide means whereby the feed-back voltage may be automatically reversed upon movement of the volume control to positions of still higher sensitivity.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further ob jects and advantages thereof, may best be understood by reference to the following description taken in connection with the following drawing in which Fig. 1 represents an embodiment of my invention, and Figs. 2 and 3 represent modifications thereof.

Referring to Fig. 1 of the drawing, I have represented therein at l a diode which may be the diode detector of an ordinary, radio receiver. Electromotive forces to be detected by the diode I are supplied thereto by a transformer 2. The detector operates into a load resistance comprising resistors 3 and 4 across which are connected the usual condenser 5. In parallel with the resisters 3 and 4 is a path comprising condenser 8, potentiometer 1 and resistance 8, the variable element 9 of the potentiometer being connected to the grid of an electron discharge device "I.

' the diode l.

The cathode of this discharge device is connected to a point on the bleeder resistance of the rectiher which supplies operating potential to the discharge devices of the system, this bleeder resistance comprising sections l2, I3, and I4. The anode of discharge device I0 is connected through anode coupling resistor I5 to the bleeder resistance at a point between resistances I3 and I4. Oscillations appearing in the output circuit of the discharge device I!) are supplied through a coupling condenser IE to the control grid of a second electron discharge device H, the output of which is supplied through a transformer Hi to a loud speaker l9.

Resistance 20 shunted by condenser 2| is connected between the cathode of discharge device i1 and ground for bias purposes, the anode thereof being connected through the primary of transformer l8 to the positive side of the source of operating voltage, the negative side of which is grounded as indicated at 22.

Conductors 23; one of which includes resistance 24, form a circuit extending from the output of discharge device I! to points respectively upon the two parallel paths, one of which comprises resistances 3 and 4 and the other of which comprises resistances l and 8, the respective points being between these resistances, this circuit com-v prising the feed-back circuit utilized to produce degeneration or regeneration in the system.

The two paths 3, 4, and 6, I, and 8 constitute a Wheatstone bridge, the resistances 3 and 4 being the ordinary diode load and having impressed thereon the rectified signal electromotive force, which may be. the ordinary audio voltages produced in the diode circuit of a radio receiver. The resistances in these paths 3, 4, and 6, 1, 8 are so proportioned that these paths comprise a Wheatstone bridge, the two conductors 23 being connected at diagonally opposite corners of the bridge. The cathode of the discharge device I0 is connected through resistance l2 and ground to a third diagonally opposite corner of the bridge which is likewise connected to the cathode of The grid of discharge device I0 is connected to a tap 9 on potentiometer i whereby this connection may be moved along resistance 1. When it is in its lower position a smallportion of the voltage produced by the diode l is supplied to the discharge device but a maximum amount of the voltage supplied by the feed-back conductors 23 is'supplied to the grid of the discharge device. As the contact! is moved upward the portion of the voltage produced by the diode which is supplied to the grid of the discharge proportioning I 9 upon resistance 1 where zero dio amplifiers are extremely important by reason of their improvement of the frequency characteristics of the amplifier to which they are applied, their reduction of hum, and their correction of distortion. Since the degeneration however, has the effect of reducing the sensitivity of the amplifier, or of reducing the amplification of the discharge device, it is important that the degeneration be completely removed when maximum amplification is required as upon reception of extremely weak signals in the radio receiver. At such times the contact 9 is moved to its upper position in order that all of the audio signal electromotive force available may be supplied to the discharge device, In accordance with my invention in which the diode load 3, 4 and the potentiometer circuits 6, 1, B are connected with respect to the diode and to the feed-back circuit in bridge relation, this feedback voltage may be reduced to zero when maximum amplification is required.

In fact, it has been found that by properly the bridge, the point of contact feed-back voltage is supplied to the grid, may be adjusted to a point short of the top of the resistor; that is,

to a point intermediate the ends of the resistor. In this way, as contact 9 is moved upward from the bottom of the resistor, the degenerative voltage is reduced until a point is reached where it is zero. Upon further movement of the contact 9 upward this voltage is reversed in phase and becomes regenerative so that it tends to increase v the amplification of the amplifier by reason of regeneration. Such increased amplification which occurs, at adjustments of the contact on the potentiometer utilized only upon reception of weak signals is, of course, valuable. In such an arrangement I have employed a resistance 3 of 3,900,000 ohms; resistance 1 of 220 ohms, re-

sistance I of 2,000,000 ohms, and resistance 8, of

22 ohms.

It will be seen that these results are secured without any appreciable complication ofthe circuits. The only resistance added to that of the i system of the above referred to Bachman application is the resistance 4 which may be of ten ohms; that is, I have utilized a ten-ohm resistor in this position in a system where resistance 3 was one of 200,000 ohms, resistance 1 of 2,000,000 ohms, and resistance 8 of 100 ohms for exact balance at top of potentiometer By reason of the balance of the bridge, as thus explained, no degenerative voltage from conductor 23 exists across the diode branch of the bridge This is very desirable since such voltage would react on the diode causing it to operate at a difierent level and would result in distortion.

comprises the cathode of the audio amplifier plus an additional anode 26. Also the amplifier in this case obtains its bias through the resistor 21 from a suitably negative point 49 of the circuit, which is by-passed by condenser 28. The condenser 39 prevents resistors I and 8 from having any efiect on this bias. The remaining amplifier I have designated on the drawing by a rectangle bearing the legend audio amplifier." v

It may be desirable at times in the application of my invention to remove the direct cur rent of the diode detector load from the bridge.

Accordingly, in Fig. 3 the diode load comprises a resistance 30, separate and apart from the bridge. The alternating current voltage of this resistance is supplied through coupling condenser 31 to the two parallel paths of the bridge comprising resistances 3, 4, and I, 8. Feed-back conductors 23 are connected to the bridge in the usual way. Resistances 32 and 33 comprise the usual bleeder resistance connected across the usual power supply conductors 35. The resistance 36 operates to maintain the grid of thedischarge device ID at negative potential with respect to its cathode. Potential is also supplied through resistances 32 and 36, conductor 31 and diode load resistance 30 to conductor 38 by which it may be supplied to the grids of the radio frequency amplifier of the preceding circuits of the receiver for normal bias purposes, this bias, of course, being increased by the potential on resistance 30, when signals are received, for automatic volume control purposes.

Since the diode I and discharge device [9 have their cathodes connected to the same point it is obvious that they may be replaced by a single combination diode and amplifier tube having a common cathode.

While I have shown particular embodiments of my invention, it will, of course, be understood that I do not wish to be limited thereto since different modifications may be made both in the circuit elements and in the instrumentalities employed and I contemplate by the appended claims to cover any such modifications as fall iv'vithinthe true spirit and scope of my invenion.

What I claim as new and desire to secure by Letters Patent of the United States:

1. In combination, an electron discharge de vice having input electrodes, and an output circuit, a Wheatstone bridge, means to supply electromotive force to be transmitted through said discharge device to one pair of diagonally opposite corners of said bridge, and means tosupply electromotive force from said output circuit to another pair of diagonally opposite comers of said bridge, the input electrodes of said discharge device being connected between one of said first pair of corners and an arm of said bridge.

2. In combination, an electron discharge device having input electrodes, and an output cirsite corners of said bridge, and means to supply The magnitude of resistance 24, of course, de-

electromotive force from said output circuit to another pair of diagonally opposite comers of said bridge, one of said input electrodes being connected to one of said first diagonally opposite comers of said bridge and the other input electrode being connected to a'variable point on an arm of said bridge, said point being movable between one of said second pairs of diagonally opposite corners and the opposite one of said first mentioned diagonally opposite corners.

3. In combination, an electron discharge device having input electrodes, a potentiometer, means to supply signal oscillations across said potentiometer, said input electrodes being variably tapped from said-potentiometer, thereby to vary the portion of said signal oscillations on said potentiometer supplied to said input electrodes, an output circuit, means to supply voltage from said output circuit to said input electrodes to produce degeneration, and means to produce said degeneration when said potentiometer is adjusted at one position and to produce regeneration when said potentiometer is adjusted at another position. I

4. In combination, an electron discharge de vice having input electrodes, a source of electromotive force to be transmitted through said discharge device, a volume control potentiometer, said input electrodes being connected across a portion of said variable potentiometer to vary the electromotive force of said source supplied to said discharge device, and means to supply voltage from the output circuit to said input circuit in degenerative phase when said input electrodes are connected across a small portion of said potentiometer and in regenerative phase when said input electrodes are connected across a large portion of said potentiometer.

5. In combination, a source of electromotive force, an electron discharge device having an input electrode connected to one point on said source and another input electrode connected to a point variable along said source to vary the electromotive force applied between said electrodes, an output circuit for said discharge device; means to supply electromotive force from said output circuit between said input electrodes, and means responsive to movement of said variable point along said source to reverse the phase of said electromotive force supplied from said output circuit as applied between said input electrodes.

6. In combination, a source of electromotive force, two parallel paths connected thereacross, an electron discharge device having an output circuit, a connection between twointermediate points on said paths, input electrodes for said electron discharge device, one of said input electrodes being connected to one side of said source, and the other input electrode being connected to a point movable along the portion of one of said paths adjacent the opposite side of said source, and means to supply voltage to said connection from said output circuit, said voltage being supplied in degenerative phase to one of said points and in regenerative phase to the other of said points.

7. In combination a source of electromotive force, two parallel paths connected thereacross,

an electron discharge device having an output circuit connected between two intermediate points respectively on said paths, and having input electrodes, one of said input electrodes being connected to one side 01' said source, and the other input electrode being connected to a point movable along the portion of one of said paths adjacent the opposite side of said source whereby the portion of the electromotive force of said source supplied between said electrodes is variable by movement 01 said point along said path, the phase of the voltage supplied from said out put circuit being degenerative as supplied between said electrodes, and said paths being so proportioned that no voltage from the output circuit appears between said electrodes when the point to which said other electrode is connected is such that a large portion of the voltage of said source is supplied to said input circuit.

8. In combination, a source of electromotive force, two parallel paths connected thereacross, an electron discharge device having an output circuit connected between two intermediate points respectively on said paths, and having input electrodes, one of said input electrodes being connected to one side of said source, the other input electrode being connected to a point movable along the portion of one of saidpaths adjacent the opposite side of said source whereby the portion of the electromotive force of said' source supplied between said electrodes is variable by movement of said point along said path, the phase of the voltage supplied from said output circuit being degenerative as supplied between said electrodes, and said paths being so proportioned that no voltage from the output circuit appears between said electrodes when the point to which said other electrode is connected is such that a large portion of the voltage of said source is supplied to said input circuit, and is in regenerative phase when said point is moved to increase the portion of the electromotive force of said source supplied between said electrodes.

9. In combination, a diode detector, an audio amplifier to amplify signals detected by said diode detector and a Wheatstone bridge con- I said bridge whereby said voltage supplied from said output circuit is impressed on said amplifier input in degenerative phase and is prevented from aflecting said diode by reason of the balance of said bridge.

10. In combination, a diode detector, an electron discharge amplifier to amplify signals detected by said detector, and having a cathode and input electrode, a bridge having two diagonal branches, one of said branches including said diode detector, and the other of said branches including the output of said amplifier, the cathode of said amplifier being connected to the terminus of said one branch adjacent the cathode of said diode and said input electrode being connected to an intermediate point on an arm of said bridge whereby voltage from the output of said amplifier is supplied to said input and is prevented from affecting said diode detector by reason of the balance of said bridge and said ca hodes are at the same potential.

CHARLES S. ROOT. 

